1. Field of the Invention
The present invention relates to a charging method and apparatus for nickel batteries having a nickel positive electrode with nickel oxide as an active material, and more particularly to improvement of the input characteristics of nickel batteries.
2. Description of the Related Art(s)
Heretofore, various secondary batteries having different characteristics are known, among them an appropriate battery type is employed according to the purpose of the application. That is to say, the output and input power level and capacity of a battery are the most important characteristics for secondary batteries, and these characteristics differ greatly according to the purpose of the application for the battery. For example, hybrid electric vehicles (HEV) require batteries with high power input and output but low capacity.
Hybrid electric vehicles operate small internal combustion engines (ICE) at a point of maximum efficiency to yield a constant average power. The operation of this sort of engine at optimum efficiency both improves fuel efficiency and reduces the amount of exhaust gas emissions. Furthermore, sufficient treatment of the exhaust gas emissions can be performed at a subsequent stage until the pollutants in the exhaust gas emissions reach a small and fixed amount, thereby enabling the emission of pollutants to be kept to a minimum.
The secondary battery to be used in hybrid electric vehicles has the following two objectives.
(1) To furnish the necessary power for the extra demand, such as during acceleration or hill climbing, when the required driving power exceeds the average output power of the engine. On the other hand, when the required driving power is less then the average output of the engine, the battery accepts the output power of the engine for charging. Therefore, the driving of the engine is maintained at an optimum point. For this reason, an output performance of high discharge power is required for the battery.
(2) To increase the energy efficiency of the vehicle through the use of regenerative braking during deceleration by converting the braking force into electrical energy for use in charging the battery. For this reason, acceptability of high charging power is also required for the battery.
To achieve these objectives, the battery for the hybrid electric vehicle must be capable, at any time during vehicle travel, of outputting and accepting high electric power. Thus, the state of charge (SOC) of the secondary battery for the hybrid electric vehicle is initially set to approximately 50%. As a result, the secondary battery is charged or discharged according to the output state of the driving power, such as for acceleration or deceleration of the vehicle. Generally, since an overcharged or overdischarged state must be avoided for the secondary battery, charging and discharging are controlled so that the SOC falls approximately within 20% to 80%.
To increase the output performance and electric power acceptability of the battery, the capacity of the battery should be increased. However, hybrid electric vehicles carry a dynamo with the initial objective of reducing the capacity of the battery, and the basic requirement is to reduce the capacity of the battery as much as possible. Therefore, it is desirable for the battery in the hybrid electric vehicle to have a small capacity and to be capable of high electric power input and output.
Nickel hydride batteries are a known type of secondary battery. The nickel hydride battery uses a positive electrode of nickel oxide and nickel hydroxide as the active materials and a negative electrode of hydrogen occluding alloy. The nickel hydride battery has high electric power output and electric power acceptability relative to its capacity, which is considered preferable as a battery for hybrid electric vehicles. Furthermore, nickel hydride batteries basically do not emit gases and are maintenance free. In this specification, the term nickel battery is understood to mean any battery using nickel oxide as the active material, such as nickel hydride batteries, nickel cadmium batteries, and so forth.
The nickel hydride battery is disclosed, for example, in Japanese Patent Laid-Open Publication No. Sho 8-124596.
In this manner, the nickel hydride battery is the preferred battery for hybrid electric vehicles. However, when various types of tests were actually conducted using nickel hydride batteries in hybrid electric vehicles, it was found that the acceptability of charging current was less than the demand. In the application of nickel hydride batteries in various types of electrical apparatus, the charging efficiency does not present much of a problem. In other words, some electric power loss during charging does not present much of a problem since the accurate state of charge during charging is not necessary to know and the battery is overcharged with electric power from commercial power sources. However, any loss during charging in hybrid electric vehicle batteries makes it difficult to control the battery SOC to be operated in predetermined SOC range. Therefore, there remains a problem where it is desirable to improve the acceptability of charging current for the battery.